Shape memory materials are materials that revert to an original shape once reaching a transition temperature. They typically exhibit high moduli of elasticity and high yield strengths above said transition temperature and comparatively low moduli and low yield strengths below the transition temperature. Such materials have been used to make coiled springs, as illustrated in U.S. Pat. No. 6,427,712; leaf springs, as shown in U.S. Pat. No. 6,436,223 and hollow tube springs, as shown in U.S. Pat. No. 5,226,979. Shape memory materials have been used in downhole tool applications to move components that actuate the tool directly in response to an energy input usually from a heating element. Some examples of downhole tool applications are U.S. Pat. Nos. 5,199,497; 6,216,799 and 6,349,767. Sometimes more than one device operated by shape memory material can be incorporated into a single tool. U.S. Pat. No. 6,433,991 shows a tool that uses a first shape memory device to move a sleeve and a second independently operated shape memory device to act as a ratchet that locks the just shifted sleeve into a position that the first device just moved it to.
Despite all these applications of shape memory materials, the present invention presents a different perspective on their use in downhole applications. Many downhole tools use a bias force to set. These tools have to be assembled at the surface using specialized equipment to pre-compress springs in retaining bores and hold the springs in that position as the tool is being assembled. There are risks to this procedure in that the spring may come loose and potentially cause injury to assembly personnel when the springs take off as projectiles. The springs can also fail during the assembly process where they are pre-compressed sending pieces into the air as projectiles that again can injure assembly personnel. Apart from the safety issues during assembly, there are the operational issues that arise from the way temperature is used to change phases of shape memory materials. If heat is added artificially, then the tool has to be configured to contain a heating element in close proximity to the shape memory material so that the desired phase change is accomplished. On the other hand, if the phase change in the shape memory material is to be initiated using the surrounding well fluid, prior applications that simply use exposure to such fluid could be prone to actuation due to prolonged exposures above the transition temperature well before the tool reaches the desired location.
The present invention addresses these issues. In the preferred embodiment of springs in a downhole tool, it provides for use of a shape memory material for the springs. The tool length can be shortened using such material for the springs in that they can be compressed to greater percentages of their free length than standard spring materials. The low modulus of shape memory materials means less force is needed to pre-compress them during assembly thus removing some level of risk from the assembly process. Also in the preferred embodiment, the crossing over the transition temperature simply energizes the springs without setting the tool. Some independent act is still undertaken to set the tool as to allow the tool to be properly positioned in the well before it is set as opposed to simply setting it when the transition temperature is crossed. These and other aspects of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment and associated drawings, while the full scope of the invention can be found from a review of the appended claims.